Traditional biomass particulate dryers employ external heat sources, such as gas-fired burners, to heat biomass particulates within a metal drum to a temperature sufficiently high to evaporate water from the particles. The water vapor is drawn out of the drum as steam. To ensure that the particulate biomass material is continually exposed to the heat, such dryers can include paddles or a helical screw auger that continuously stirs the biomass within the drum. Alternatively, the drum may be rotated to agitate the particulates. Using such systems, it is possible to dry wet sawdust from a moisture content of up to 90%, achieving a moisture content as low as about 10%. However, considerable fuel is burned to provide the heat for drying the particulate biomass in a conventional drum dryer, and more energy is required to rotate the drum or the internal agitating mechanism. Unless the heat applied is waste heat from some other productive process, the drying of particulate biomass materials can be a relatively expensive process, particularly due to the increasing cost of fossil fuels.
Conventional dryers implement what can be characterized as a batch drying process. The drum of a conventional dryer is typically loaded with a charge of wet biomass particulate material and the heat from the external source is applied until the desired moisture content of the material being dried is achieved. One type of biomass material that must be dried is wet sawdust, which may be produced at a lumber mill as logs are sawn into lumber, rail ties, or some other type of wood product. Lumber mills process logs on a continuous basis while in operation, so the sawdust that is a byproduct of the sawing operation is produced continually. Ideally, it would be desirable to dry the sawdust on a continuous basis so that the resulting dried wood particles used to make wood pellet fuel and animal bedding, pressed wood logs, and other products is also being produced on a continuous basis. Accordingly, it will be evident that it would be more desirable to provide a biomass drying system that can dry biomass particulates on a continuous basis, producing an output stream of dried wood particles for further product production. The speed and efficiency of the drying process would thus be greatly enhanced by providing a continuous feed process biomass dryer.
Another characteristic of conventional biomass drum dryers is that they are typically installed as fixed systems and are sized to handle batches of biomass material of a desired volume. Accordingly, for applications in which there is a need for a portable biomass dryer, the conventional systems are typically not practical. Also, the amount of biomass material that must be processed can sometimes be variable. For example, if the source of biomass material produces volumes of the wet material that vary substantially, it can be even less efficient to run a relatively smaller charge of the material through a conventional externally heated drum dryer when the volume to be processed is smaller than the design volume of the drum. Thus, another benefit of a continuous processing biomass dryer would be that the processing might simply be halted once the available mass of biomass material has been dried.
Furthermore, drum dryers are not suitable for drying some of the waste streams produced by various industries. Specifically, waste materials having a characteristic small particulate size cannot normally be processed in drum dryers. These materials include sludge from waste water treatment plants, spent grains from ethanol productions facilities, wet waste paper from paper mills, waste pulp, and a host of other similar materials. It would therefore be desirable to provide a dryer that can be employed to dry such materials, so that they can be used as alternative fuels instead of being put in land fill or burned wet with the added heat provided by a secondary fuel source. Because a suitable dryer is not available, many of the producers of these waste streams are putting them in land fills at a substantial expense to themselves, and causing an adverse impact on the environment.
Since the conventional biomass dryers are unable to overcome the problems noted above, it would clearly be desirable to develop a biomass dryer that operates in a substantially different manner that is able to provide continuous batch processing and is more portable. While the amount of biomass material that is to be processed is less of an issue in a continuous processing system, it would still be desirable to provide a continuous process biomass dryer that can readily be sized for almost any desired throughput rate, so that the processing capability can be generally matched to the maximum required throughput rate. The biomass dryer should also be generally portable, so that it can readily be moved to a site where there is a need for the dryer.